They were gone.
Looking at the remains of the Potters' home, Peter knew that they had put up a fight, and had been killed. And it was all his fault.
I didn't perform the curse, his guilty mind protested weakly. I didn't really kill them.
His conscience (which sounded suspiciously like Remus) disagreed. You handed them over to him. It amounts to the same thing.
He wandered numbly through the wreckage. He knelt down and shut James's eyes, unable to bear the sight of him staring lifelessly at Peter. He continued upstairs, up to Harry's room. But where a little nursery had been only a few days ago, now there was just destruction. He saw Lily's body lying on the floor, and shut her eyes as well.
He picked up Voldemort's wand and put it in his pocket, unaware of even doing so. Later he would discover it in his pocket, bitterly noting that cowardly self-preservation had become instinctive for him now.
Feeling overwhelmed by the death and destruction he had caused he Apparated, not caring where he went. He ended up on a mostly deserted Muggle street (he had never bothered to learn their names), completely intact except for the index finger on his right hand, which had been splinched due to his lack of focus. He sat down in the street, muttering, "They're gone. All gone, all my fault. He probably has Harry now. All my fault. They're gone." He was dimly aware that some of the Muggles were looking at him strangely, but he didn't care. How could he worry about what a few people thought of him when they were gone?
"Peter! Peter! You treacherous little Squib!" a voice called out, fury and grief making his voice shake. "How could you Peter? You betrayed them!"
Naturally, Sirius found me, Peter thought. He must have gone to my hiding place, seen that I wasn't there, and figured it out. But why? Now I have to betray him too.
"No, you betrayed them! You gave them over to the Dark Lord, Sirius! They were your friends, how could you?" Peter's guilt and fear caused him to accidentally use magic: several of the surrounding buildings exploded, possibly killing the Muggles on the street; he'd never know for sure. Taking advantage of the chaos, he morphed into a rat and scurried into the sewers. The sound of his old friend laughing maniacally followed him, and caused another pang of guilt; he knew he'd pushed Sirius to brink of insanity, and quite possibly over it.
Peter felt tremendous guilt, but as always, his fear was stronger than his guilt. Only now, his fear was of seeing Lily and James in the afterlife. They would surely shun him and hate him. And they should, Peter thought sadly. After all, a rat is all I am. All I'll ever be.